In ITS, systems that provide information available for avoiding accidents such as early discovery of an oncoming car on a road with poor visibility, rapid notice that a car is entering a blind crossing, and the like, have been investigated. As a means for realizing such a system, a configuration in which a terminal apparatus capable of performing radio communication is set on each car so that data such as the position, speed, etc. of each car is notified to others by means of the terminal apparatuses, can be considered.
At present, in Japan, two frequency bands, namely 5.8 GHz band and 700 MHz band have been studied as the radio frequencies for ITS. As to the apparatuses for dedicated short-range communications (DSRC: Dedicated Short-Range Communication) to be used for ITS, proposals have been presented by Japanese Laid-Open Patent Application No. 2003-203292A and others, for example.
Typically, when the frequency band used for radio communication is low, the number of links available for communications in that frequency band decreases. On the other hand, since cars that are equipped with a terminal apparatus for ITS will tend to increase in number, it is preferable that usage efficiency of the frequency band be improved so as to offer the communication service to a greater number of ITS users.
Radio waves of a relatively high frequency band of about a few gigahertz travels in a highly straight direction and have greater decay rates in space. Therefore, if such radio waves in the high frequency band are used for radio communication, it is difficult to establish communication with a terminal apparatus that is set in a car that is located far away from the subject car, or a terminal apparatus that is set in a car that is located at a range invisible to the subject car, i.e., at a position blocked by buildings or the like, for example.
In contrast to this, radio waves of a relatively low frequency band of about some hundreds of megahertz have smaller decay rates in space and present a large ratio of diffraction. Accordingly, if radio waves in such a low-frequency band are used for radio communication, it is even possible to establish communication with a terminal apparatus that is set on a car that is not visible to the subject car. However, if radio waves of low frequency bands are used for radio communication, the communication area becomes wider so that the number of communicable terminal apparatuses increases. As a result, the subject car will communicate with not only oncoming traffic on the aforementioned road with poor visibility or incoming cars entering a blind crossing, but also with the terminal apparatuses set on the cars other than these. Accordingly, a greater amount of information than is necessary is given to the user of the ITS.